The present invention relates to the liquid phase oxidation of aromatic hydrocarbons in the presence of at least one heavy metal oxidation catalyst and bromine, which is activated by anthracene or another polycyclic aromatic compound to produce aromatic carboxylic acids. The present invention includes the liquid phase oxidation of pseudocumene (PSC) (1,2,4-trimethylbenzene) in the presence of a catalyst comprising a multivalent catalyst, a source of bromine, and a polycyclic aromatic hydrocarbon, to produce trimellitic acid (TMLA). The present invention relates to the liquid phase oxidation of PSC in the presence of a catalyst comprising a multivalent metal oxidation catalyst, a source of bromine, and a polycyclic aromatic hydrocarbon selected from anthracene, naphthalene, tetracene, and combinations thereof to produce TMLA. Trimellitic acid can be dehydrated to produce trimellitic anhydride (TMA). TMA and TMLA are commercially valuable as the raw materials for manufacture of polyester materials. Trimellitate esters are used as plasticizers for polyvinyl chloride, especially for high performance wire and cable insulation as these have principle features of temperature stability and low volatility. Trimellitic anhydride is used in the production of resins for electrodeposition and powder coatings, and as a binder for glass fibers, sand, and other aggregates. Trimellitic anhydride is used as an embossing agent for vinyl flooring and as a curing agent for epoxy resins. It is also used as an intermediate for the synthesis of surface coatings chemicals, adhesives, polymers, dyes printing inks, pharmaceuticals and agrochemicals.
Aromatic carboxylic acids such as benzene dicarboxylic acids and naphthalene dicarboxylic acids are commercially valuable as the raw materials for manufacture of polyester materials which are used to manufacture fibers, films, resins, and many other petrochemical compounds. U.S. Pat. No. 2,833,816, hereby incorporated by reference, discloses the liquid phase oxidation of xylene isomers into corresponding benzene dicarboxylic acids in the presence of bromine using a catalyst having cobalt and manganese components. As described in U.S. Pat. No. 5,183,933 incorporated by reference herein in its entirety, liquid phase oxidation of dimethylnaphthalenes to naphthalene dicarboxylic acids can also be accomplished in the presence of bromine and a catalyst having cobalt and manganese components. Typically, aromatic carboxylic acids are purified in a subsequent process as described, for example, in U.S. Pat. No. 3,584,039, U.S. Pat. No. 4,892,972, and U.S. Pat. No. 5,362,908.
The liquid phase oxidation of aromatic hydrocarbons to aromatic carboxylic acids is conducted using a reaction mixture comprising aromatic hydrocarbons and a solvent. Typically, the solvent comprises a C1-C8 monocarboxylic acid, for example acetic acid, benzoic acid, or mixtures thereof with water. As used herein, “aromatic hydrocarbon” preferably means a molecule composed predominantly of carbon atoms and hydrogen atoms, and having one or more aromatic rings, particularly dimethyl benzenes, trimethyl benzenes, and dimethyl naphthalenes. Aromatic hydrocarbons suitable for liquid-phase oxidation to produce aromatic carboxylic acid generally comprise an aromatic hydrocarbon having at least one substituent group that is oxidizable to a carboxylic acid group. As used herein, “aromatic carboxylic acid” preferably means an aromatic hydrocarbon with at least one carboxyl group.
A bromine promoter and catalyst are added to the reaction mixture which is reacted in the presence of an oxidant gas. Typically, the catalyst comprises at least one suitable heavy metal component. Suitable heavy metals include heavy metals with atomic weight in the range of about 23 to about 178. Examples include cobalt, manganese, vanadium, molybdenum, nickel, zirconium, titanium, cerium or a lanthanide metal such as hafnium. Suitable forms of these metals include for example, acetates, hydroxides, and carbonates. The use of bromine in producing aromatic carboxylic acids by liquid phase oxidation improves conversion of the reactants.
USSR patent no. 239936 (I. V. Zakharov) discloses a method for the liquid-phase oxidation of alkyl-aromatic hydrocarbons with molecular oxygen in an acetic-acid solution in the presence of a catalyst—a cobalt salt and dibromoanthracene—at a temperature of 90-110° C., wherein, for the purpose of intensifying the process, a manganese salt addition in a volume of 1-3% of the cobalt salt concentration is introduced into the reaction mixture.
Quality of aromatic carboxylic acids is often determined by the concentration of intermediate products found as impurities in the aromatic carboxylic acid product. The type and concentration of these impurities varies with the types and concentrations of catalyst and promoter used and with the particular aromatic carboxylic acid product desired. The presence of such impurities may interfere with use of the carboxylic acid product or make it less desirable for certain purposes. For example, when terephthalic acid is used in a direct condensation process in preparing polyesters, impurities in the terephthalic acid can cause undesirable coloration of the polyester and can act as chain terminators.
It has been discovered that anthracene and other polycyclic aromatic hydrocarbons activate the oxidations of alkylaromatics to aromatic carboxylic acids even when added in very small amounts. This activation is reflected in increased oxygen uptake, temperature increases, lower intermediates and shortened reaction time and higher yield of primary product.
The addition of anthracene, naphthalene and other polycyclic aromatic hydrocarbons to the oxidation of alkylaromatics, such as, xylenes, trimethylbenzenes and dimethylnaphthalenes causes an unexpected and pronounced activation which can enhance the production of aromatic acids such as terephthalic acid (TA), isophthalic acid (IPA), trimellitic acid (TMLA), and naphthalene dicarboxylic acid (NDA). Higher activities in these oxidations (catalyzed by Co, Mn and Br) can lead to reduced intermediates and by-products, lower catalyst costs and reduced corrosion and emissions caused by Br. Very small levels of anthracene or other polycyclic aromatic hydrocarbon are needed to cause this activation. Using anthracene or another polycyclic aromatic hydrocarbon as an activator may reduce catalyst costs by enabling one to obtain good conversion of the starting aromatic hydrocarbon to the desired aromatic carboxylic acid with less catalyst metal. Being able to use less cobalt, for example, can produce a significant cost savings in the process.
The activation of the oxidation of aromatic hydrocarbons to aromatic carboxylic acids with polycyclic aromatics, such as anthracene, could result in significant decreases in catalyst concentration which would reduce catalyst cost significantly, especially if the amount of cobalt, which is the costliest component in the catalyst packages, can be decreased. The ability to use less catalyst is an unexpected advantage which can provide cost savings and a more economical process. This provides a particular cost saving advantage in those processes where recovery and recycling of expensive catalyst components, such as cobalt is difficult or not possible.
In addition, the use of anthracene to activate the oxidation of aromatic hydrocarbons to aromatic carboxylic acids may permit the oxidation process to be run at a lower temperature which means that less energy would have to be used in the process. This could also provide a cost savings and, in addition, using less energy is desirable from an environmental standpoint.
Another difficulty encountered in the liquid phase oxidation of aromatic hydrocarbons to form aromatic carboxylic acids is solvent and aromatic hydrocarbon burning. The liquid phase oxidation reaction typically results in the burning of at least 2% of the solvent and more than 2% of the aromatic hydrocarbon. We have discovered that the use of a polycyclic aromatic hydrocarbon as a promoter increases the yield of product aromatic carboxylic acid without detrimental increase in solvent and hydrocarbon burning.